1. Field of the Invention
The invention relates to the accurate detection of a moving sheet of glass. It proposes a mechanical device for detecting the fact that a sheet of glass carried on a conveyor has passed a given point The detection device according to the invention is particularly directed at installations for the heat treatment of forming of sheets of glass.
2. Background of the Prior Art
The majority of treatments carried out on sheets of glass can only be performed properly if the glass sheet is located in perfect registery and is stationary at a precise point in the treatment chain.
For instance, in order to give sheets of glass a curvature of given radius, it is usual to heat them individually in a tunnel furnace, transport them on a conveyor and then transfer them from the conveyor to a bending frame or bending press. The trickiest part of this operation is effecting the transfer without making any visible marks or faults on glass which has been raised to its softening temperature.
Many authors have given their attention to improving the transfer means. In all cases, the transfer is carried out in the following successive stages: the sheet of glass which is being transported on a horizontal conveyor is immobilized, then moved vertically by vacuum means and/or raised by a gaseous pressure, after which the bending apparatus is located under the sheet of glass. The sheet of glass then falls--possibly under control--and is then taken over by the said bending apparatus.
A good final positioning of the glass sheet is possible only if its immobilization at the first stage is perfectly controlled, which means that the conveyor has to stop abruptly with the sheet under or, according to the circumstances, above the apparatus which performs the vertical movements of the sheet. To avoid any sliding of the sheet of glass, it is all that part of the conveyor which is above the level of the vertical displacement means which has to be stopped simultaneously. Since on the other hand, even with highly automated units, the sheet processing rate is never perfectly regular, it is necessary to detect the passage of the glass at a point on the conveyor, for instance shortly after the exit from the tunnel furnace, in order to control with an adequate time lag the actual stopping of the conveyor.
The choice of detection system generally falls to devices which entail no material contact between the glass and the detector, so that there is no risk of causing defects. However, the applicants have found that visual detectors are not sufficiently viable under the temperature conditions which obtain in a bending chamber. Indeed, at a temperature of around 700.degree. C., the light rays are considerably diffracted and above all any slight variation in the temperature in the chamber alters the angle of diffraction so that the ray is no longer detected.
Establishing this fact has led to the applicants departing completely from the prior art and propose an essentially mechanical detector which is unaffected by the ambient temperature.